1. Field of the Invention
This invention relates to a reverse painting process which is suitable for painting a vehicle body.
2. Description of the Related Art
There is a method which comprises forming a powder coating on, for example, a part of a vehicle body, giving it heat treatment, and forming an electrodeposited coating on the rest thereof. This method is called reverse painting, and has been drawing attention as a method of painting a vehicle body owing to, for example, a thick film which can easily be formed by powder coating. There have been proposed a number of reverse painting processes as disclosed in (1) Japanese Patent Publication No. SHO 56-10397 entitled xe2x80x9cA Process for Painting an Automobile Bodyxe2x80x9d, and (2) Japanese Patent Publication No. SHO 62-2640 entitled xe2x80x9cA Reverse Painting Processxe2x80x9d.
FIG. 7 hereof is a chart showing a conventional reverse painting process, or the process as disclosed in (1). It comprises applying a powdery paint to the outer plate of an automobile body, melting it by heat to form a film thereon, coating the inner plate of the body with a film formed by electrodeposition, and baking both of the films together to cure them by heat.
FIG. 8 hereof is a chart showing another conventional reverse painting process, or the process as disclosed in (2). It comprises applying a powdery paint to a part of an article to be painted, fusing it by heat to form a film thereon, coating the other part of the article with a film formed by electrodeposition, washing the films with water, drying them, and baking them together. If they are baked immediately after washing with water, water marks are likely to form. Therefore, the process includes the drying step for evaporating water, so that no water mark may form.
The processes as disclosed in the prior Japanese patent publications (1) and (2) have the steps of powder coating, fusing, and electrodeposition in common. Referring more particularly to the fusing step, FIG. 9 is a graph showing the fusing temperature and time as employed in the conventional processes, and shows the temperature (xc2x0 C.) along the ordinate axis and the time (minutes) along the abscissa axis.
Referring more particularly to FIG. 9, a vehicle body remaining at room temperature (15xc2x0 C.) is coated with a powdery paint, and placed in a fusing oven. The vehicle body is composed of a skin, sash, beam, hinge, etc., and while the skin having the smallest thickness of all is heated in a short time, the hinge having the largest thickness of all is not heated quickly. Therefore, the vehicle body is heated in the oven for 36 minutes until the fusion of the paint on the hinge is completed. The skin is necessarily held at a high temperature for a long time.
FIGS. 10(a) to 10(d) are a series of diagrams showing a conventional pattern of reverse painting. FIG. 10(a) shows a multiplicity of coating powder particles 101 applied to a part of the upper surface of a skin 100 and held adhering to it by an electric attracting force, and a number of coating powder particles 101a scattered on the skin near the right end of a powder coating area A adjacent to an electrodeposition area B. If they are heated, the particles 101 are partly melted together, while the particles 101a are shaped like bells, as shown in FIG. 10(b).
If they are further heated, a substantially flat coating film 102 is formed on the skin in the powder coating area A except in the vicinity of its end portion where the particles 101a shown in FIG. 10(b) form films 101b shaped like low hills, as shown in FIG. 10(c). Then, electrodeposition is carried out to cover any portion of the skin not covered with the film 101b or 102. As a result, shallot-shaped films 105 are formed in a boundary area between the film 102 formed by powder coating and a flat film 104 formed by electrodeposition, as shown in FIG. 10(d).
Each hill-shaped film 101b has, however, a very thin portion having a thickness of only, say, 5 microns which is too small for any satisfactory rustproofness. The boundary area is, therefore, undesirably low in rustproofness.
It is, therefore, an object of this invention to provide a painting process which can improve the rustproofness of the boundary between an area of powder coating and an area of electrodeposition.
As a result of our review of the prior art, we, the inventors of this invention, have found that the small thickness of the films 101b which is responsible for the undesirably low rustproofness of the boundary area is due to the excessive deformation by heat of the particles in that area, and that such deformation is due to the prolonged exposure of the skin to a high temperature as required for heating the hinge having a larger heat capacity. We have, therefore, considered that the shortening of the time for heating the skin will be an effective solution to the outstanding problem.
We have thought of the local heating of the hinge and beam, but found it difficult to realize in any ordinary heating oven. We have, therefore, thought of preheating a vehicle body to be painted, and found that the skin is cooled soon after preheating, though the hinge, etc. having a larger heat capacity are not cooled soon, and that if the vehicle body is placed in a fusing oven at a proper timing while the hinge, etc. are still hot, it is possible to heat the hinge, etc. to an appropriate fusing temperature and thereby shorten the time which is required for the adequate heating of the vehicle body as a whole.
FIG. 1 is a chart showing a reverse painting process embodying this invention and including, after the preliminary step of washing a piece of work to be painted, the steps of preheating the work, while evaporating any water remaining on it after washing, applying a powdery paint to the preheated work, heating it in a fusing oven to form a coating film on the work, coating the work with a film formed by electrodeposition, and baking both of those films. Although FIG. 1 includes the preliminary step of washing, it and the step of evaporating any water (or the drying step) can be carried out in a separate line, so that the process according to this invention may start with the preheating step.
FIGS. 2(a) to 2(c) show a pattern of reverse painting according to the process embodying this invention. FIG. 2(a) shows coating powder particles 2 applied to a part of the upper surface of a skin 1 and held adhering to it by an electric attracting force, and coating powder particles 2a scattered on the skin near the right end of a powder coating area A adjacent to an electrodeposition area B. After only a short time of heating, the particles 2a are slightly melted and adhere to the skin 1, as shown in FIG. 2(b). If electrodeposition is, then, carried out to form a film 5, films 6 are also formed in the boundary area between a film 3 formed from the particles 2 and the film 5 so as to fill the open spaces left around the bell-shaped particles 2a, as shown in FIG. 2(c). The bell-shaped particles 2a have a thickness D2 which is sufficiently large for forming a rustproof film in the boundary area.
According to a first aspect of this invention, there is, thus, provided a reverse painting process which comprises the steps of preheating the work to be painted, applying a powdery paint to the work upon cooling of the work to below the crosslinking and curing temperature of the paint, melting the paint by heat at a temperature not below its softening point, but below its crosslinking and curing temperature to form a film on the work in a fusing oven before the work is cooled to ordinary temperature, and coating the work with a film formed by electrodeposition.
Provision of the work preheating step makes it possible to form a film having a sufficiently large thickness for maintaining a satisfactorily high level of rustproofness in the boundary area between a film of the powdery paint and a film formed by electrodeposition, thus achieving drastic shortening of the time required for heating it in the fusing oven.
According to a second aspect of this invention, there is provided a reverse painting process which comprises the steps of washing the work to be painted, preheating the work, while evaporating any water remaining on it after washing, applying a powdery paint to the work upon cooling of the work to below the crosslinking and curing temperature of the paint, melting the paint by heat at a temperature not below its softening point, but below its crosslinking and curing temperature to form a film on the work in a fusing oven before the work is cooled to ordinary temperature, and coating the work with a film formed by electrodeposition.
The preheating of the work can be performed in a drying oven which is used for evaporating any water from the work after washing. If any existing drying oven can be used, it is possible to cut down the cost of equipment for carrying out the process of this invention accordingly.
Moreover, provision of the work preheating step makes it possible to form a film having a sufficiently large thickness for maintaining a satisfactorily high level of rustproofness in the boundary area between a film of the powdery paint and a film formed by electrodeposition, thus achieving drastic shortening of the time required for heating it in the fusing oven.